1. Field of the Invention
The present invention relates to a compound semiconductor device for use in power amplifiers or others of movable communication terminals.
2. Description of the Related Art
In terminals of communication movable bodies, such as portable telephones, etc. MESFETs using GaAs, which are capable of high-speed operation, are used as parts of power amplifiers of the transmission units.
In the conventional MESFETs, however, a threshold voltage is lower than 0 V, and even with a gate voltage set at 0 V, drain current is not completely turned off, and a trace of drain leak current of tenth-order .mu.A flows. Even a trace of drain leak current is a cause for short battery life for portable telephones, which are long in waiting state without communication (waiting time).
The drain leak current of the MESFET can be made null by applying a high negative voltage to the gate electrode, but to this end separate batteries for generating a negative voltage are necessary. Mounting a plurality of batteries causes a volume increase and a cost increase unpreferably in terms of properties of the movable body terminal.
In place of mounting extra batteries for generating the negative voltage, it can be proposed that a DC/DC converter is mounted for generating the negative voltage, but this is not preferable because more current than the drain leak current is consumed by the DC/DC converter.
It has therefore been proposed that a switch transistor which operates at a positive voltage should be inserted in the drain terminal of the MESFET, whereby the drain current is shut off.
However, when the switch transistor operates, and a current flows between the collector and the emitter, a voltage decrease takes place between the collector and the emitter, and the voltage decrease becomes larger as the flowing current is increased. Accordingly, a voltage to be applied to the drain electrode of the MESFET adversely lowers. To maintain the same output electric power as in the prior art, a voltage decrease must be supplied by increasing current. In addition, to decrease electric power consumption of the terminal of the communication movable body, the electric power source voltage is lowered, which makes the voltage decrease of the switch transistor more serious.
Therefore, a compound semiconductor device which does not require insertion of a switch transistor has been proposed.
The proposed compound semiconductor device will be explained with reference to FIG. 5. FIG. 5 is a sectional view of the proposed compound semiconductor device.
In the proposed compound semiconductor device, a thin barrier layer is used to facilitate influence of the gate voltage on the electron transit layer to thereby make the threshold voltage higher than 0 V.
A 600 nm-thickness buffer layer 212 of GaAs is formed on a semi-insulating GaAs substrate 210, and a 14 nm-thickness electron transit layer 214 of In.sub.0.2 Ga.sub.0.8 As is formed on the buffer layer 212.
A 20 nm-thickness barrier layer 218 of Al.sub.0.75 Ga.sub.0.25 As is formed on the electron transit layer 214. Because there is a tendency that a decrease of a thickness of the barrier layer 218 increases the gate leak current, the barrier layer 218 is formed of a material having a high Al composition ratio to prevent increase of the gate leak current.
A 30 nm-thickness GaAs layer 220 is formed on the barrier layer 218 for good contact with the respective electrodes.
A gate electrode 222 is formed on the GaAs layer 220, and a source electrode 224 and a drain electrode 226 are formed on both sides of the gate electrode 222.
In regions below the source electrode 224 and the drain electrode 226, ohmic regions 228, 230 heavily doped with an n-impurity are formed down to the semi-insulating GaAs substrate 210.
For preventing concentration of an electric field between the ohmic regions 228, 230 and the region undoped with the impurity, LDD (Lightly Doped Drain) regions 232, 234 which reach the semi-insulating GaAs substrate 210 are formed in the region other than the region below the gate electrode 222.
As described above, the proposed semiconductor device includes the barrier layer 218 having the high Al composition ratio and a small thickness, whereby the threshold voltage can be greater than 0 V, so that the drain current can be OFF without applying a negative voltage to the gate electrode.
However, the above-described proposed compound semiconductor device, which includes the barrier layer 218 having the high Al composition ratio on the Al-free electron transit layer 214, has such a poor junction of the interface between the electron transit layer 214 and the barrier layer 218 that the barrier layer 218 has many crystal defects. As a result, the proposed compound semiconductor device unpractically has a small gain and low reliability.